The present invention relates generally to systems and methods for measuring transmissivity of transparencies, and more particularly to a novel field usable device for making accurate measurements of transmissivity.
Previous methods for measuring optical transmissivity of a transparency generally comprise placing a source and detector on opposite sides of the transparency to measure transmitted light. The method described in ASTM D1003-61 measures luminous transmittance through the transparency using an integrating sphere, and requires accurate alignment of source and detector. A second method utilizes a diffuse area light source of uniform luminance which is measured first directly and then through the transparency using a photometer, the transmissivity being calculated as a ratio of the luminance measured with the transparency in the path to the luminance of the source without the transparency. A difficulty with this procedure is that unwanted reflections from the transparency may cause erroneous readings. Each method requires relatively expensive equipment and a trained operator to obtain accurate results.
The invention described herein meets the need for a simple, accurate, inexpensive, field useable device to measure transmissivity of transparencies in aircraft windscreens, automotive windshields and the like. The invention includes a diffuse light source of controllable luminance for placement against one side of the transparency to be measured, and a detector for placement against the other side of the transparency, the detector including a photo diode, cadmium sulfide cell or similar detector element spaced a preselected distance from the aperture to detect only light from a preselected solid cone angle originating from a point near the detector. Although a lens may be included in the aperture of the detector in order to better collect transmitted light to the detector element, the absence of any lens in a preferred form of the invention significantly simplifies structure and use of the invention. The combination of a relatively large uniform light source and a relatively small recessed light detector element provides a condition similar to an infinite extent light source for a limited distance between light source and detector. As the distance between detector and light source increases, the area of the diffuse light source that illuminates the detector increases as the square of the distance to the detector. At the same time, the distance from any point on the surface of the light source to the surface of the detector also increases such that the illumination contribution from each light source point is decreasing with the square of the distance. These two effects exactly compensate and illumination reaching the detector remains constant throughout the distance for which these conditions pertain. This condition only occurs if the light source is a true Lambertian (substantially totally diffusing) source. For a diffuse light source of given area, any transparency with an optical thickness that falls within this constant illumination range may therefore be measured without resorting to different thickness calibration transparencies.
It is therefore a principal object of the invention to provide an improved transmissivity measuring device and method.
It is another object of the invention to provide an accurate field usable device and method for measuring transmissivity of transparencies.
These and other objects of the invention will become apparent as the detailed description of representative embodiments proceeds.